Since an electrophotographic technique achieves instant image-formation and provides images of high quality, the electrophotographic technique has been recently widely used not only in the field of copying machines but also in the field of various kinds of printers.
As the photoreceptor which is an essential member of the electrophotographic technique, photoreceptors using organic photoconductive materials (hereinafter referred to as organic photoreceptors) having advantages of no pollution problem, easy film-formation, easy production thereof, etc., have been recently developed in place of the inorganic photoconductors such as selenium, an arsenic-selenium alloy, cadmium sulfide, zinc oxide, etc., which have hitherto been used as the photoconductive materials.
In the organic photoreceptors, a laminated layer type photoreceptor comprising a charge generating layer and a charge transfer layer laminated each other is developed and has mainly been subjected to the investigations.
The laminated layer type photoreceptor has a high possibility of becoming the main subject of photoreceptors and has been positively developed, because the photoreceptor having a high sensitivity can be obtained by combining a charge generating layer and a charge transport layer each having a high efficiency, the photoreceptor having a wide selective range of materials and having a high safety can be obtained, the productivity of layer coating is high, and the photoreceptor is relatively advantageous in cost.
However, the laminated layer type photoreceptors which have hitherto been practically used have various problems in the electric characteristics that the light sensitivity is insufficient, the residual electric potential is high, and the light responsive property is poor. Further, it suffers problems upon repeated use that the charging property is lowered, the residual electrostatic charges are accumulated, the sensitivity is deviated, etc. The conventional laminated layer type photoreceptors therefore could not have sufficient characteristics. In these problems, the deterioration caused by repeated use of the photoreceptor, i.e., the deterioration of the charging property and the sensitivity caused by the increase of the residual potential, the wear of the photosensitive layer by the abrasion of the layer in the cleaning step in the electrophotographic process, etc., directly causes lowering of the image quality, whereby such a laminated layer type photoreceptor does not have a sufficient printing durability at present. Accordingly, in order to use an organic photoreceptor for an electrophotographic process of high speed, it is very important in the practical use for increasing the reliability of the copying machine to always form stable images through compensation of the image quality deterioration due to the deterioration of the photoreceptor by controlling the electrophotographic process.
Examples of such a process controlling method include a method of timely detecting the surface potential of the photoreceptor by setting a surface electrometer in a copying machine and optimally controlling the output of the elecrtostatically charging device and the voltage of the copy lamp according to the result of the detection; and a method of forming a latent image of standard white on a photoreceptor, developing the latent image thus printed with a toner, detecting the density of the toner image by an optical sensor, and optimally controlling the output of the electrostatic charging device, the toner concentration of the developer, the developing bias potential, and the copy lamp voltage according to the result of the detection.
However, when the latter method was attempted for example, a sufficient image was not obtained. That is, the above-mentioned conditions were practically controlled to try to obtain stable images by forming a toner image of a definite area (e.g., 10 mm.times.10 mm) on the surface of a photoreceptor, correctly measuring the change of the reflection density, determining the extent of deterioration of the photoreceptor by comparing the measured result with the initial value, and feeding back the result to the charging electric potential, the developing bias electric potential, etc. However, even when toner images each having a definite area were formed on the surface of a cylindrical photoreceptor under a same condition, the deviation of the reflection density became large, whereby a constant value was not obtained and it was difficult to sufficiently correct the images.
The reason is considered to be as follows: When a cylindrical photoreceptor is used, each toner image is not formed at the same position since the process starting position is located at an unspecified position on the photoreceptor in each process, thereby the distances between the surface of the photoreceptor and the processing units, such as the electrostatic charger, the sensor for detection and the developing roller, are changed in each position for forming the toner image due to the rotating deflection of the center axis for rotating the cylindrical photoreceptor, the tolerance of the mechanical dimensions of the cylindrical photoreceptor itself, and the rotation tolerance of the developing roller, and thus the reflection density is also changed.
For carrying out such a control process effectively, it is necessary at least to form each toner image at a definite position on the photoreceptor to keep a constant distance between the surface of the photoreceptor and each process unit. While there may be many means for detecting the specific position of the surface of the photoreceptor, examples thereof include a method of applying a marking to a rotating member corresponding to the rotation of a cylindrical photoreceptor and reading the marking with a sensor, and a method of applying a marking to the photoreceptor itself and reading the marking with a sensor. In any cases, in order to carry out the process control with a high reliability, it is necessary to make a marking such that the marking portion can be detected with high accuracy.